Elastic wave device

ABSTRACT

There is disclosed an acoustic wave apparatus, constructed in such a manner that a surface rotated in the range of 34° to 41° from a crystal Y axis around the crystal X axis of lithium tantalate is set as the surface of a substrate, a standardized electrode thickness (h/λ) obtained by standardizing a thickness h of an electrode finger constituting at least a part of an interdigital transducer by a wavelength λ of a surface acoustic wave is set to the range of 0.01 to 0.05, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP 00/07239 which has an-Internationalfiling date of Oct. 18, 2000, which designated the United States ofAmerica and was not published in English.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an acoustic wave apparatus forpropagating acoustic waves, used for the circuit of a communicationequipment, an electronic device or the like.

2. Background Art

Heretofore, in such an acoustic wave apparatus in which a piezoelectricsubstrate containing lithium tantalate (LiTaO₃, referred to as LThereinafter) has been used, the cut angle θ of the LT substrate has beenset equal to 36°. This setting was a result of the calculation that ifan electrode was formed on the surface of such a substrate, and thesubstrate surface was electrically short-circuited, the amount ofpropagation loss would be reduced to nearly a value of zero.

However, such calculation was made by assuming the establishment of anideal state where the electrode had no thickness. Consequently, in theactual acoustic wave apparatus comprising an electrode having thickness,there was a possibility that a condition for reducing the amount ofpropagation loss to a minimum may be different. In addition, thecalculation was made by examining the case where the entire surface ofthe substrate was covered with the electrode. Consequently, in theacoustic wave apparatus comprising electrode fingers cyclically arrayedas in the case of an SAW filter, there was a possibility that acondition for reducing the amount of propagation loss to a minimum mightbe different.

Thus, in Japanese Patent Application Laid-Open No. 1997-167936 (referredto as a document 1, hereinafter), a condition for reducing the amount ofpropagation loss to a minimum is examined by taking into considerationthe thickness of a grating electrode formed on the surface of the LTsubstrate. FIG. 1 shows the result of calculating the amount ofpropagation loss in a ladder surface acoustic wave filter of thedocument 1 shown in FIG. 7. In the drawing, an ordinate indicates theamount of loss made when a surface acoustic wave (referred to as SAW,hereinafter) is propagated per wavelength (λ), i.e., the amount of lossper wavelength (dB/λ). An abscissa indicates a standardized electrodethickness (h/λ), where the thickness h of the electrode is standardizedbased on the wavelength λ of SAW.

FIG. 1 shows the case where an LT crystal X-axis direction is set as aSAW propagation direction, a surface perpendicular to a “θ-rotated Y”axis obtained by rotating a crystal Y axis by θ around the crystal Xaxis, is set as a substrate surface, and a cut angle θ is set in therange of 36° to 46°. The LT substrate having the surface perpendicularto the “θ-rotated Y” axis set as its surface and the crystal X-axisdirection set as the SAW propagation direction is represented byθ-rotated Y-cut X-propagation lithium tantalate, abbreviated to θYX-LT,or θYX-LiTaO₃. In many cases, the electrode is made of aluminum (Al) oran alloy mainly containing Al.

As shown in FIG. 1, if a standardized electrode thickness (h/λ) is θ,the amount of loss per wavelength (dB/λ) is minimum when a cut angle θis about 36°. This result coincides with that of the conventionalcalculation, i.e., if the ideal state of the electrode having nothickness is established, the amount of propagation loss is reduced tonearly a value of zero when a cut angle θ is 36°.

In addition, as shown in FIG. 1, if a cut angle θ is 40°, the amount ofloss per wavelength (dB/λ) is minimum when a standardized electrodethickness (h/λ) is about 0.05. If a cut angle θ is 42°, the amount ofloss per wavelength (dB/λ) is minimum when a standardized electrodethickness (h/λ) is about 0.075. Accordingly, in the SAW device realizedby setting the standardized electrode thickness (h/λ) in a range above0.05, a cut angle θ for reducing the amount of propagation loss to aminimum resides in a range above 40°.

As apparent from the foregoing discussion made with reference to FIG. 1,it is possible to reduce the amount of propagation loss to a minimum byselecting the proper combination of a standardized electrode thickness(h/λ) with a cut angle θ. As a result, the insertion loss of the SAWdevice can be reduced. Therefore, in recent years, the LT substratehaving a cut angle θ set equal to 42° has been employed.

There are several kinds of acoustic waves. If a cut angle θ is set inthe range of about 36° to 46°, and the direction of propagation is acrystal X axis, for example, a surface skimming bulk wave (SSBW), whichis a bulk wave propagated along the surface of an LT substrate describedin a document: pp. 158-165, “Journal of Institute of Electronics andCommunication Engineers of Japan”, Vo 1. J67-C, No. 1, January 1984(referred to as a document 2, hereinafter), and a leaky surface acousticwave (LSAW) are propagated. In the present application, these waves aregenerically termed as SAW, except when the waves are distinguished fromeach other.

FIG. 2 is an upper surface view showing the constitution of the SAWfilter, which is one type of an acoustic wave apparatus. In the drawing,a reference numeral 1 denotes an LT substrate made of a piezoelectricmaterial; 3 an electrode finger; 4 a bonding pad; 5 an input sideinterdigital transducer (IDT) for performing electric—surface acousticwave energy conversion; 6 an output side IDT for performing surfaceacoustic wave—electric energy conversion; 7 an input terminal; and an 8an output terminal. W 0 represents a maximum value of the length of aportion intersected by the electrode finger 3.

FIG. 3 is a sectional view of the SAW filter shown in FIG. 2. In thedrawing, a code w represents a width of the electrode finger 3; p anarraying cycle of electrode fingers 3; and h a thickness of theelectrode finger 3.

Next, the operation of the SAW filter will be described.

An electric signal applied to the input terminal 7 forms an electricfield at the intersection of each electrode finger 3 of the input sideIDT 5. In this case, as the LT substrate 1 is made of the piezoelectricmaterial, the electric field causes distortion. If the input signal hasa frequency f, the strain that has been generated is vibrated at thefrequency f, converting the signal into SAW. This SAW is thentransmitted in a direction perpendicular to the electrode finger 3. Atthe output side IDT 6, the SAW is converted back into the electricsignal. The conversion of the electric signal into the SAW, and theconversion of the SAW into the electric signal are reversible to eachother.

If a cut angle θ is about 36°, and the propagation direction of the SAWis in a crystal X-axis direction, as described in the document 2, thedisplacement component of the SAW has a direction parallel to theelectrode finger 3, and the surface of the LT substrate 1. Such adisplacement component depends on the cut angle θ of the cut surface ofthe LT substrate 1 and the propagation direction of the SAW.

The SAW excited by the input side IDT 5 is propagated toward the outputside IDT 6. However, if there is propagation loss in the LT substrate 1,the power of the SAW having reached the output side IDT 6 is smallerthan that of the SAW immediately after its excitation by the input sideIDT 5. The amount of the loss is approximately equal to a value obtainedby multiplying a distance between the centers of the input side IDT 5and the output side IDT 6 standardized based on the wavelength λ of theSAW by an attenuation constant α.

Thus, assuming that the distances of the input side IDT 5 and the outputside IDT 6 are equal to each other, as the amount of propagation loss inthe LT substrate 1 is increased, the amount of insertion loss for theSAW filter is larger. As described in a document: pp. 56 to 81, “SurfaceAcoustic Wave Engineering”, Institute of Electronics and CommunicationEngineers of Japan, issued by Corona Inc., November 1983, since thewavelength λ of the SAW is double the arraying cycle p of the electrodefingers 3, the amount of loss generated following propagation isapproximately equal to a value, which is obtained by multiplying anumerical value half an average value of the numbers of electrodefingers 3 constituting the input side IDT 5 and the output side IDT 6 byan attenuation constant α.

For example, as shown in FIG. 2, assuming that each of the input sideIDT 5 and the output side IDT 6 has 7 electrode fingers 3, and the inputside IDT 5 and the output side IDT 6 are disposed close to each other,the amount of loss generated following propagation is equal to a value,which is about 3 to 4 times larger than the attenuation constant α. Asan example, if an attenuation constant α is 0.02 (dB/λ), then the amountof loss following propagation takes a value set in the range of 0.06 to0.08 dB.

As apparent from the foregoing, in order to realize a low-loss SAWdevice, it is important to use an LT substrate 1 having a small amountof propagation loss. Heretofore, in the acoustic wave apparatus of theforegoing type, a cut angle θ set in a range above 36° has beenemployed.

As described above, the propagation loss greatly affects the insertionloss of the SAW filter. However, the propagation loss is not the onlyfactor that affects the insertion loss of the SAW filter. As materialconstants for representing the characteristics of the LT substrate 1, inaddition to the propagation loss, there are an electromechanicalcoupling coefficient k² regarding conversion efficiency between theelectric signal and the SAW, a capacitance C0 regarding the impedance ofthe input or output side IDT 5 or 6, the propagation velocity Vs of theSAW, and so on. Among these constants, the electromechanical couplingcoefficient k² is particularly important, because it decides theinsertion loss or the pass band width of the SAW filter.

For the acoustic wave apparatus using a pure surface acoustic wavebringing about no propagation loss in principle, such as a Rayleighwave, Bleustein-Gulyaev-Shimizu (BGS) wave or the like, optimaldesigning conditions were known. However, for the acoustic waveapparatus using LSAW or SSBW, no specific conditions were known.

As described above, the conventional acoustic wave apparatus of theforegoing type has been used under the condition for minimizing thepropagation loss. However, since the electromechanical couplingcoefficient k² for greatly affecting the characteristics of the acousticwave apparatus has not been used under any optimal conditions,deterioration has inevitably occurred in the insertion loss or the bandwidth of the acoustic wave apparatus.

The present invention was made to solve the foregoing problems, and itis an object of the invention to provide an acoustic wave apparatus withlower loss characteristics and wider band than the conventional acousticwave apparatus of the foregoing type.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided an acousticwave apparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; and an interdigital transducer including a conductorformed on the substrate. In this case, a surface rotated in the range of34° to 41° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as the surface of the substrate, a standardizedelectrode thickness (h/λ) obtained by standardizing a thickness h of anelectrode finger constituting the interdigital transducer by awavelength λ of a surface acoustic wave is set in the range of 0.01 to0.05, and a duty ratio (w/p) of the electrode finger decided based on awidth w and an arraying cycle p of the electrode finger is set to thevalue ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; and an interdigital transducer including a conductorformed on the substrate. In this case, a surface rotated in the range of35° to 42° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as the surface of the substrate, a standardizedelectrode thickness (h/λ) obtained by standardizing a thickness h of anelectrode finger constituting the interdigital transducer by awavelength λ of a surface acoustic wave is set in the range of 0.05 to0.075, and a duty ratio (w/p) of the electrode finger decided based on awidth w and an arraying cycle p of the electrode finger is set to thevalue ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; and an interdigital transducer including a conductorformed on the substrate. In this case, a surface rotated in the range of36° to 43° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting the interdigital transducer by a wavelength λ of asurface acoustic wave is set in the range of 0.075 to 0.1, and a dutyratio (w/p) of the electrode finger decided based on a width w and anarraying cycle p of the electrode finger is set to the value rangingfrom 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band than the conventional apparatus can be realized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; an interdigital transducer including a conductorformed on the substrate; and a reflector including a conductor formed onthe substrate. In this case, a surface rotated in the range of 34° to41° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting at least a part of the reflector by a wavelength λof a surface acoustic wave is set in the range of 0.01 to 0.05, and aduty ratio (w/p) of the electrode finger decided based on a width w andan arraying cycle p of the electrode finger is set to the value rangingfrom 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; an interdigital transducer including a conductorformed on the substrate; and a reflector including a conductor formed onthe substrate. In this case, a surface rotated in the range of 35° to42° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting at least a part of said reflector by a wavelength λof a surface acoustic wave is set in the range of 0.05 to 0.075, and aduty ratio (w/p) of the electrode finger decided based on a width w andan arraying cycle p of the electrode finger is set to the value rangingfrom 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; an interdigital transducer including a conductorformed on the substrate; and a reflector including a conductor formed onthe substrate. In this case, a surface rotated in the range of 36° to43° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting at least a part of the reflector by a wavelength λof a surface acoustic wave is set in the range of 0.075 to 0.1, and aduty ratio (w/p) of the electrode finger decided based on a width w andan arraying cycle p of the electrode finger is set to the value rangingfrom 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; and an interdigital transducer including a conductorformed on the substrate. In this case, a surface rotated in the range of34° to 41° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting a part of the interdigital transducer by awavelength λ of a surface acoustic wave is set in the range of 0.01 to0.05, and a duty ratio (w/p) of the electrode finger decided based on awidth w and an arraying cycle p of the electrode finger is set to thevalue ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; and an interdigital transducer including a conductorformed on the substrate. In this case, a surface rotated in the range of35° to 42° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting a part of the interdigital transducer by awavelength λ of a surface acoustic wave is set in the range of 0.05 to0.75, and a duty ratio (w/p) of the electrode finger decided based on awidth w and an arraying cycle p of the electrode finger is set to thevalue ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; and an interdigital transducer including a conductorformed on the substrate. In this case, a surface rotated in the range of36° to 43° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting a part of the interdigital transducer by awavelength λ of a surface acoustic wave is set in the range of 0.075 to0.1, and a duty ratio (w/p) of the electrode finger decided based on awidth w and an arraying cycle p of the electrode finger is set to thevalue ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; an interdigital transducer including a conductorformed on the substrate; and a reflector including a conductor formed onthe substrate. In this case, a surface rotated in the range of 34° to41° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of a part of anelectrode finger constituting a part of the reflector by a wavelength λof a surface acoustic wave is set in the range of 0.01 to 0.05, and aduty ratio (w/p) of a part of the electrode finger decided based on awidth w and an arraying cycle p of a part of the electrode finger is setto the value ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; an interdigital transducer including a conductorformed on the substrate; and a reflector including a conductor formed onthe substrate. In this case, a surface rotated in the range of 35° to42° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of a part of anelectrode finger constituting a part of the reflector by a wavelength λof a surface acoustic wave is set in the range of 0.05 to 0.075, and aduty ratio (w/p) of a part of the electrode finger decided based on awidth w and an arranging cycle of a part of the electrode finger is setto the value ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

In accordance with the invention, there is provided an acoustic waveapparatus, comprising: a piezoelectric substrate mainly containinglithium tantalate; an interdigital transducer including a conductorformed on the substrate; and a reflector including a conductor formed onthe substrate. In this case, a surface rotated in the range of 36° to43° from a crystal Y axis around a crystal X axis of the lithiumtantalate is set as a surface of the substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of a part of anelectrode finger constituting a part of the reflector by a wavelength λof a surface acoustic wave is set in the range of 0.075 to 0.1, and aduty ratio (w/p) of a part of the electrode finger decided based on awidth w and an arraying cycle p of a part of the electrode finger is setto the value ranging from 0.6 to just below 1.0.

Thus, an acoustic wave apparatus with lower loss characteristics andwider band characteristics than the conventional apparatus can berealized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a result of calculating an attenuationconstant, described in Japanese Patent Application Laid-Open No.1997-167936.

FIG. 2 is an upper surface view showing a constitution of an SAW filter.

FIG. 3 is a sectional view of the SAW filter shown in FIG. 2.

FIG. 4 is a view showing a result of calculating an attenuation constantwhen a standardized electrode thickness is set equal to 0.01, accordingto a first embodiment of the present invention.

FIG. 5 is a view showing a result of calculating an electromechanicalcoupling coefficient when the standardized electrode thickness is equalto 0.01, according to the first embodiment of the invention.

FIG. 6 is a view showing a result of calculating an attenuation constantwhen a standardized electrode thickness is set equal to 0.05, accordingto the first embodiment of the invention.

FIG. 7 is a view showing a result of calculating an electromechanicalcoupling coefficient when the standardized electrode thickness is equalto 0.05, according to the first embodiment of the invention.

FIG. 8 is a view showing a result of calculating an attenuation constantwhen a standardized electrode thickness is set equal to 0.075, accordingto the first embodiment of the invention.

FIG. 9 is a view showing a result of calculating an electromechanicalcoupling coefficient when the standardized electrode thickness is equalto 0.075, according to the first embodiment of the invention.

FIG. 10 is a view showing a result of calculating an attenuationconstant in an LT substrate when a standardized electrode thickness isset equal to 0.1, according to the first embodiment of the invention.

FIG. 11 is a view showing a result of calculating an electromechanicalcoupling coefficient in the LT substrate when the standardized electrodethickness is equal to 0.1, according to the first embodiment of theinvention.

FIG. 12 is a view showing a pattern of a mode-coupled SAW resonantfilter.

FIG. 13 is a view showing a result of calculating a minimum insertionloss value of the mode-coupled SAW resonant filter shown in FIG. 12.

FIG. 14 is a view showing a result of calculating feedthrough power ofthe mode-coupled SAW resonant filter shown in FIG. 12.

BEST MODE FOR CARRYING OUT THE INVENTION

Several embodiments for carrying out best the present invention are nowexplained in detail with reference to attached drawings.

[First Embodiment]

FIG. 4 is a view showing the result of calculating an attenuationconstant α when a standardized electrode thickness (h/λ) is 0.01. In thedrawing, an ordinate indicates an attenuation constant α (dB/λ); and anabscissa a cut angle θ of an LT substrate. As in the case shown in FIG.1, the crystal X-axis direction of the LT substrate is set as thepropagation direction of SAW, and a surface perpendicular to the axis ofrotating a crystal Y axis by θ around the crystal X axis, i.e., thesurface of rotating the crystal Y axis by θ around the crystal X axis,is set as the surface of the LT substrate.

In the described case, calculation is made as to the propagationcharacteristic of SAW when electrode fingers each having a width w likethat shown in FIG. 3 are endlessly arrayed at an arraying cycle p. FIG.4 specifically shows the results of calculation made by changing theduty ratio (w/p) of the electrode finger decided based on the width wand the arraying cycle p of the electrode finger from 0.2 to 0.8 each by0.1.

FIG. 5 is a view showing the result of calculating an electromechanicalcoupling coefficient k² when a standardized electrode thickness (h/λ) is0.01. In the drawing, an ordinate indicates an electromechanicalcoupling coefficient k²; and an abscissa a cut angle θ of the LTsubstrate as in the case shown in FIG. 4. FIG. 5 shows the results ofcalculation made by changing the duty ratio (w/p) based on the samevalues as those shown in FIG. 4.

The results of calculation shown in FIGS. 4 and 5 are based on themethods of analysis using discrete Green function respectively describedin, for example, a document: pp. 649 to 654, “Recent Studies on AcousticWave Device Technology-Committee Report-, March, 1995, by Acoustic WaveDevice Technology 150th Committee of Japan Society for the Promotion ofScience (referred to as a document 4, hereinafter), a document: pp. 786to 791 of the same (document 5, hereinafter), and a document: pp. 93 to100, 23rd EM Symposium, May, 1994 (document 6, hereinafter). The backscattering effect of a program (FEMSDA) described in the document 5 isexcluded in the results of calculation.

Similarly to FIGS. 4 and 5, FIGS. 6 and 7 show the results ofcalculation each made when a standardized electrode thickness (h/λ) is0.05. The value 0.05 of the standardized electrode thickness (h/λ) isused relatively frequently in the SAW device of a GHz band.

Referring to FIG. 6, a cut angle θ with respect to a minimum attenuationconstant α is shown to be larger than that shown in FIG. 4. In FIG. 7,however, if a cut angle θ is selected for a smaller attenuation constantα, an electromechanical coupling coefficient k² is smaller. In otherwords, as shown in FIG. 6, an attenuation constant α takes a minimumvalue when a cut angle θ is about 38°. But an electromechanical couplingcoefficient k² takes a larger value when a cut angle θ is lower than38°.

Similarly to FIGS. 4 and 5, or FIGS. 6 and 7, FIGS. 8 and 9 show theresults of calculation each made when a standardized electrode thickness(h/λ) is 0.075. Referring to FIG. 8, a cut angle θ with respect to aminimum attenuation constant α is shown to be larger than that shown inFIG. 6. An attenuation constant α is minimum when a cut angle θ is about39°. In FIG. 9, however, if a cut angle θ is selected for a smallerattenuation constant α, an electromechanical coupling coefficient k² issmaller. In other words, an electromechanical coupling coefficient k²takes a larger value when a cut angle θ is lower than 39°.

Similarly to FIGS. 4 and 5, FIGS. 6 and 7, or FIGS. 8 and 9, FIGS. 10and 11 show the results of calculation each made when a standardizedelectrode thickness (h/λ) is 0.1. Referring to FIG. 10, a cut angle θwith respect to a minimum attenuation constant α is shown to be largerthan that shown in FIG. 8. An attenuation constant α is minimum when acut angle θ is about 40°. In FIG. 11, however, if a cut angle θ isselected for a smaller attenuation constant α, an electromechanicalcoupling constant k² is smaller. In other words, an electromechanicalcoupling coefficient k² takes a larger value when a cut angle θ is lowerthan 40°.

In FIG. 6, for example, if a duty ratio (w/p) is 0.5, an attenuationconstant α takes a minimum value substantially equal to 0 when a cutangle θ is about 38°. However, in the case of the calculation result forthe conventional acoustic wave apparatus of such a type, shown in FIG.1, if a standardized electrode thickness (h/λ) is 0.05, an attenuationconstant α takes a minimum value equal to 0 when a cut angle θ is about40°. This is attributed to the fact that the calculation result of FIG.1 is for the ladder surface acoustic wave filter according to thedocument 1, and is different from the calculation result for theacoustic wave apparatus constructed by arraying the electrode fingers atan endless cycle.

FIG. 12 shows the pattern of a mode-coupled SAW resonant filter, used todetermine the effects of the calculation results shown in FIGS. 4 to 11on the SAW filter. In the drawing, a reference numeral 5 denotes aninput side IDT. There are 31 electrode fingers 3 provided. A referencenumeral 6 denotes an output side IDT. Two output sides IDT 6 areelectrically connected in parallel with each other. One side of theoutput side IDT 6 has 18 electrode fingers 3, while the other side has20 electrode fingers 3. A reference numeral 9 denotes a gratingreflector, which has 120 electrode fingers (i.e., strips) 10 in oneside. The line widths of the electrode fingers 3 of the input and outputside IDT 5 and 6 are all equal to one another at wi, and also arrayingcycles all equal to one another at pi.

The arraying cycle pg of the electrode fingers 10 of the gratingreflector 9 shown in FIG. 12 is set at pg=1.0251 pi, different fromthose of the input and output side IDT 5 and 6. The duty ratio (wg/pg)of the electrode fingers 10 of the grating reflector 9 is equal to that(wi/pi) of the electrode fingers 3 of each of the input and output sideIDT 5 and 6. Hereinafter, these duty ratios (wi/pi) and (wg/pg) aregenerically referred to as a duty ratio (w/p).

Distances D1 and D2 are respectively 2.5 pi and 0.25 pi. A maximumintersection width is 360 μm.

FIG. 13 shows the result of calculating a minimum insertion loss valuefor the SAW filter shown in FIG. 12. Specifically, FIG. 13 shows theresults of calculation made by changing duty ratios (w/p) from 0.5 to0.7 each by 0.1, when a standardized electrode thickness (h/λ) is 0.05for each of the electrode fingers 3 and 10 respectively of the input andoutput side IDT 5 and 6 and the grating reflector 9.

For the calculation, for example, a document: pp. 185 to 205, “AcousticWave Device Technology Handbook”, November 1991, by Acoustic Wave DeviceTechnology 150th Committee of Japan Society for the Promotion of Science(referred to as a document 7, hereinafter) is available. In thedescribed case, specifically, a 2nd equivalent circuit model by Smithdescribed therein (FIG. 3.76, p. 188 of the document 7) is used for theinput and output side IDT 5 and 6.

For the grating reflector 9, for example, a document: pp. 206 to 227,“Acoustic Wave Device Technology Handbook”, November 1991, by AcousticWave Device Technology 150th Committee of Japan Society for thePromotion of Science (referred to as a document 8, hereinafter) isavailable. In the described case, specifically, a distributed constantequivalent circuit described therein (right side of FIG. 3.134, p. 221of the document 8) is used.

The change of a cut angle θ or a duty ratio (w/p) causes a change in thepropagation velocity Vs of SAW. In the described case, however,calculation is made by changing an arraying cycle pi in such a way as toset a center frequency f0 of each of the input and output side IDT 5 and6 equal to 965 MHz. In addition, the calculation is made by taking intoconsideration not only the changes of an attenuation constant α and anelectromechanical coupling coefficient k², but also the changes ofmaterial constants for the SAW propagation velocity Vs, a capacitanceC0, a reflection coefficient C1, and so on.

Referring to FIG. 13, a value of insertion loss is shown to be minimumwhen a cut angle θ is about 38°, which is smaller than that used in theconventional acoustic wave apparatus of such a type when a cut angle θis 36° or 42°. Referring to FIG. 6, an attenuation constant α is shownto be minimum when a cut angle θ is 38°. This explains why the value ofinsertion loss is minimum when the cut angle θ is about 38°. It cantherefore be understood that the attenuation constant α greatly affectsthe amount of insertion loss.

FIG. 14 shows the result of calculating feedthrough power for the SAWfilter shown in FIG. 12. Specifically, FIG. 14 shows the results ofcalculation made by changing cut angles θ to 36°, 38° and 42°, when astandardized electrode thickness (h/λ) of each of the electrode fingers3 and 10 respectively of the input and output side IDT 5 and 6 and thegrating reflector 9 is 0.05, and a duty ratio (w/p) is 0.7.

In addition, the calculation is made by assuming that when a cut angle θis 36°, an acoustic velocity Vs is 4083.4 (m/s), an attenuation constantα 0.01749 (dB/λ), a capacitance C0 304 (pF/m) per electrode finger, andan electromechanical coupling coefficient k² 11.7%; when a cut angle θis 38°, an acoustic velocity Vs is 4085.6 (m/s), an attenuation constantα 8×10⁻⁶ (dB/λ), a capacitance C0 304 (pF/m) per electrode finger, andan electromechanical coupling coefficient k² 11.4%; and when a cut angleθ is 42°, an acoustic velocity Vs is 4088.3 (m/s), an attenuationconstant α 0.00833 (dB/λ), a capacitance C0 302 (pF/m) per electrodefinger, and an electromechanical coupling coefficient k² 11.0%.

As shown in FIG. 14, feedthrough power is only slightly lower when thecut angle θ is 36° compared with that when the cut angle θ is 38°.However, when the cut angle θ is 42°, feedthrough power is lower by 0.1dB or more compared with that when the cut angle θ is 38°. As shown inFIG. 6, an attenuation constant α is larger when the cut angle θ is 36°or 42°, compared with that when the cut angle θ is 38°. As shown in FIG.7, an electromechanical coupling coefficient k² is larger when the cutangle θ is 36°, compared with that when the cut angle θ is 38°. When thecut angle θ is 42°, an electromechanical coupling coefficient k² issmaller compared with that when the cut angle θ is 38°. The onlyslightly lower level of the feedthrough power when the cut angle θ is36° compared with that when the cut angle θ is 38° can be attributed tothe fact that when the cut angle θ is 36°, not only the attenuationconstant α but also the electromechanical coupling coefficient k² arelarger than those when the cut angle θ is 38°. The lower level of thefeedthrough power by 0.1 dB or more when the cut angle θ is 42° comparedwith that when the cut angle θ is 38° can be attributed to the fact thatwhen the cut angle θ is 42°, the attenuation constant α is larger thanthat when the cut angle θ is 38°, but the electromechanical couplingcoefficient k² is smaller. Accordingly, it can be understood that theelectromechanical coupling coefficient k² also affects the amount ofinsertion loss greatly.

As can be understood from FIG. 7, the increase of the duty ratio (w/p)brings about a larger electromechanical coupling coefficient k²,reducing the amount of insertion loss and widening a pass band width.Even if the larger attenuation constant α increases the amount of lossfollowing propagation, a corresponding increase if made in theelectromechanical coupling coefficient k² results in a reduction in theamount of insertion loss.

As shown in FIG. 4, the attenuation constant α takes a minimum valuewhen the cut angle θ is about 37° and, by setting the cut angle θ in therange of 34° to 40°, the attenuation constant α can be limited to 0.005(dB/λ). In addition, as shown FIG. 6, the attenuation constant α takes aminimum value when the cut angle θ is about 38° and, by setting the cutangle θ in the range of 35° to 41°, the attenuation constant α can belimited to 0.005 (dB/λ).

As shown in FIGS. 5 and 7, when the cut angle θ is set in the range of34° to 41°, the cut angle θ is 36° or 42° if the duty ratio (w/p) is 0.6or higher. The electromechanical coupling coefficient k² is larger thanthat in the conventional case having the duty ratio (w/p) of 0.5. In thecase having a large standardized electrode thickness (h/λ) shown in FIG.7, compared with that shown in FIG. 5, the electromechanical couplingcoefficient k² is larger under equal cut angles θ and equal duty ratios(w/p). There have been no reports made hitherto regarding the result ofcalculating an electromechanical coupling coefficient based onconsideration given to the duty ratio. Thus, in the described case, ageneral duty ratio 0.5 was used as a conventional duty ratio. The sameapplies hereinafter.

As described above, according to the first embodiment, the cut angle θof the LT substrate is set in the range of 34° to 41°, the electrodethickness of each of the electrode fingers 3 and 10 respectively of theinput and output side IDT 5 and 6 and the grating reflector 9 is set inthe range of 0.01 to 0.05 with respect to a wavelength of SAW, and theduty ration (w/p) of the electrode fingers 3 and 10 respectively of theinput and output side IDT 5 and 6 and the grating reflector 9 is set tothe value ranging from 0.6 to just below 1.0. Thus, it is possible torealize an acoustic wave apparatus with lower loss characteristics andwider band characteristics as compared with the conventional acousticwave apparatus of such a type.

Such an advantage can be obtained not only when the input and outputside IDT 5 and 6 and the grating reflector 9 all satisfy theabove-described conditions, but also when only one of the componentssatisfies the conditions.

For example, a similar advantage can be obtained when only the inputside IDT 5 satisfies the conditions, i.e., when the cut angle θ of theLT substrate is set in the range of 34° to 41°, the electrode thicknessof the electrode finger 3 of the input side IDT 5 is set in the range of0.01 to 0.05 with respect to a wavelength of SAW, and the duty ratio(w/p) of the electrode finger 3 of the input side IDT 5 is set to thevalue ranging from 0.6 to just below 1.0.

Similarly, a similar advantage can be obtained when only the output sideIDT 6 satisfies the conditions, i.e., when the cut angle θ of the LTsubstrate is set in the range of 34° to 41°, the electrode 3 thicknessof the electrode finger 3 of the output side IDT 6 is set in the rangeof 0.01 to 0.05 with respect to a wavelength of SAW, and the duty ratio(w/p) of the electrode finger 3 of the output side IDT 6 is set to thevalue ranging from 0.6 to just below 1.0.

Likewise, a similar advantage can be obtained when only the gratingreflector 9 satisfies the conditions, i.e., when the cut angle θ of theLT substrate is set in the range of 34° to 41°, the electrode thicknessof the electrode finger 10 of the grating reflector 9 is set in therange of 0.01 to 0.05 with respect to a wavelength of SAW, and the dutyratio (w/p) of the electrode finger 10 of the grating reflector 9 is setto the value ranging from 0.6 to just below 1.0.

In addition, the foregoing advantage can be obtained not only when allthe electrode fingers of the input and output side IDT 5 and 6 and thegrating reflector 9 satisfy the conditions, but also when a part of theelectrode fingers satisfies the conditions.

For example, a similar advantage can be obtained when a part of theelectrode fingers 3 of the input side IDT 5 satisfies the conditions,i.e., when the cut angle θ of the LT substrate is set in the range of34° to 41°, the electrode thickness of a part of the electrode fingers 3of the input side IDT 5 is set in the range of 0.01 to 0.05 with respectto a wavelength of SAW, and the duty ratio (w/p) of a part of theelectrode fingers 3 thereof is set to the value ranging from 0.6 to justbelow 1.0.

Similarly, a similar advantage can be obtained when a part of theelectrode fingers 3 of the output side IDT 6 satisfies the conditions,i.e., when the cut angle θ of the LT substrate is set in the range of34° to 41°, and the electrode thickness of a part of the electrodefingers 3 of the output side IDT 6 is set in the range of 0.01 to 0.05with respect to a wavelength of SAW, and the duty ratio (w/p) of a partof the electrode fingers 3 thereof is set to the value ranging from 0.6to just below 1.0.

Likewise, a similar advantage can be obtained when a part of theelectrode fingers 10 of the grating reflector 9 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 34° to 41°, the electrode thickness of a part of the electrodefingers 10 of the grating reflector 9 is set in the range of 0.01 to0.05 with respect to a wavelength of SAW, and the duty ratio (w/p) of apart of the electrode fingers 10 thereof is set to the value rangingfrom 0.6 to just below 1.0.

Furthermore, the foregoing advantage can obtained not only when allportions of a part of the electrode fingers of the input and output sideIDT 5 and 6 and the grating reflector 9 satisfy the conditions, but alsowhen only a portion thereof satisfies the conditions.

For example, a similar advantage can be obtained when only a portion ofa part of the electrode fingers 3 of the input side IDT 5 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 34° to 41°, the electrode thickness of a portion of a part ofthe electrode fingers 3 of the input side IDT 5 is set in the range of0.01 to 0.05 with respect to a wavelength of SAW, and the duty ratio(w/p) of a portion of a part of the electrode fingers 3 thereof is setto the value ranging from 0.6 to just below 1.0.

Similarly, a similar advantage can be obtained when only a portion of apart of the electrode fingers 3 of the output side IDT 6 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 34° to 41°, the electrode thickness of a portion of a part ofthe electrode fingers 3 of the output side IDT 6 is set in the range of0.01 to 0.05 with respect to a wavelength of SAW, and the duty ratio(w/p) of a portion of a part of the electrode fingers 3 thereof is setto the value ranging from 0.6 to just below 1.0.

Likewise, a similar advantage can be obtained when only a portion of apart of the electrode fingers 10 of the grating reflector 9 satisfiesthe conditions, i.e., when the cut angle θ of the LT substrate is set inthe range of 34° to 41°, the electrode thickness of a portion of a partof the electrode fingers 10 of the grating reflector 9 is set in therange of 0.01 to 0.05 with respect to a wavelength of SAW, and the dutyratio (w/p) of a portion of a part of the electrode fingers 10 thereofis set to the value ranging from 0.6 to just below 1.0.

[Second Embodiment]

As shown in FIG. 6, the attenuation constant α takes a minimum valuewhen the cut angle θ is about 38° and, by setting the cut angle θ in therange of 35° to 41°, the attenuation constant α can be limited to 0.005(dB/λ). In addition, as shown in FIG. 8, the attenuation constant αtakes a minimum value when the cut angle θ is about 39° and, by settingthe cut angle θ in the range of 36° to 42°, the attenuation constant αcan be limited to 0.005 (dB/λ).

As shown in FIGS. 7 and 9, when the cut angle θ is set in the range of35° to 42°, and the duty ratio (w/p) is 0.6 or higher, theelectromechanical coupling coefficient k² is larger than that in theconventional case in which the cut angle θ is 36° and the duty ratio(w/p) is 0.5. In the case having a large standardized electrodethickness (h/λ) shown in FIG. 9, compared with that shown in FIG. 7, theelectromechanical coupling coefficient k² is larger under equal cutangles θ and equal duty ratios (w/p).

As described above, according to the second embodiment, the cut angle θof the LT substrate is set in the range of 35° to 42°, the electrodethickness of each of the electrode fingers 3 and 10 respectively of theinput and output side IDT 5 and 6 and the grating reflector 9 is set inthe range of 0.05 to 0.075 with respect to a wavelength of SAW, and theduty ratio (w/p) of the electrode fingers 3 and 10 respectively of theinput and output side IDT 5 and 6 and the grating reflector 9 is set tothe value ranging from 0.6 to just below 1.0. Thus, it is possible torealize an acoustic wave apparatus with lower loss characteristics andwider band characteristics as compared with the conventional acousticwave apparatus of such a type.

Such an advantage can be obtained not only when the input and outputside IDT 5 and 6 and the grating reflector 9 all satisfy the foregoingconditions, but also when only one of the components satisfies theconditions.

For example, a similar advantage can be obtained when only the inputside IDT 5 satisfies the conditions, i.e., when the cut angle θ of theLT substrate is set in the range of 35° to 42°, the electrode thicknessof the electrode finger 3 of the input side IDT 5 is set in the range of0.05 to 0.075 with respect to a wavelength of SAW, and the duty ratio(w/p) of the electrode finger 3 of the input side IDT 5 is set to thevalue ranging from 0.6 to just below 1.0.

Similarly, a similar advantage can be obtained when only the output sideIDT 6 satisfies the conditions, i.e., when the cut angle θ of the LTsubstrate is set in the range of 35° to 42°, the electrode thickness ofthe electrode finger 3 of the output side IDT 6 is set in the range of0.05 to 0.075 with respect to a wavelength of SAW, and the duty ratio(w/p) of the electrode finger 3 of the output side IDT 6 is set to thevalue ranging from 0.6 to just below 1.0.

Likewise, a similar advantage can be obtained when only the gratingreflector 9 satisfies the conditions, i.e., when the cut angle θ of theLT substrate is set in the range of 35° to 42°, the electrode thicknessof the electrode finger 10 of the grating reflector 9 is set in therange of 0.05 to 0.07 with respect to a wavelength of SAW, and the dutyratio (w/p) of the electrode finger 10 of the grating reflector 9 is setto the value ranging from 0.6 to just below 1.0.

In addition, the foregoing advantage can be obtained not only when allthe electrode fingers of the input and output side IDT 5 and 6 and thegrating reflector 9 satisfy the conditions, but also when a part of theelectrode fingers satisfies the conditions.

For example, a similar advantage can be obtained when a part of theelectrode fingers 3 of the input side IDT 5 satisfies the conditions,i.e., when the cut angle θ of the LT substrate is set in the range of35° to 42°, the electrode thickness of a part of the electrode fingers 3of the input side IDT 5 is set in the range of 0.05 to 0.075 withrespect to a wavelength of SAW, and the duty ratio (w/p) of a part ofthe electrode fingers 3 thereof is set to the value ranging from 0.6 tojust below 1.0.

Similarly, a similar advantage can be obtained when a part of theelectrode fingers 3 of the output side IDT 6 satisfies the conditions,i.e., when the cut angle θ of the LT substrate is set in the range of35° to 42°, the electrode thickness of a part of the electrode fingers 3of the output side IDT 6 is set in the range of 0.05 to 0.075 withrespect to a wavelength of SAW, and the duty ratio (w/p) of a part ofthe electrode fingers 3 thereof is set to the value ranging from 0.6 tojust below 1.0.

Likewise, a similar advantage can be obtained when a part of theelectrode fingers 10 of the grating reflector 9 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 35° to 42°, the electrode thickness of a part of the electrodefingers 10 of the grating reflector 9 is set in the range of 0.05 to0.075 with respect to a wavelength of SAW, and the duty ratio (w/p) of apart of the electrode fingers 10 thereof is set to the value rangingfrom 0.6 to just below 1.0.

Furthermore, the foregoing advantage can be obtained not only when allportions of a part of the electrode fingers of the input and output sideIDT 5 and 6 and the grating reflector 9 satisfy the conditions, but alsowhen only a portion thereof satisfies the conditions.

For example, a similar advantage can be obtained when only a portion ofa part of the electrode fingers 3 of the input side IDT 5 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 35° to 42°, the electrode thickness of a portion of a part ofthe electrode fingers 3 of the input side IDT 5 is set in the range of0.05 to 0.075 with respect to a wavelength of SAW, and the duty ratio(w/p) of a portion of a part of the electrode fingers 3 thereof is setto the value ranging from 0.6 to just below 1.0.

Similarly, a similar advantage can be obtained when only a portion of apart of the electrode fingers 3 of the output side IDT 6 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 35° to 42°, the electrode thickness of a portion of a part ofthe electrode fingers 3 of the output side IDT 6 is set in the range of0.05 to 0.075 with respect to a wavelength of SAW, and the duty ratio(w/p) of a portion of a part of the electrode fingers 3 thereof is setto the value ranging from 0.6 to just below 1.0.

Likewise, a similar advantage can be obtained when only a portion of apart of the electrode fingers 10 of the grating reflector 9 satisfiesthe conditions, i.e., when the cut angle θ of the LT substrate is set inthe range of 35° to 42°, the electrode thickness of a portion of a partof the electrode fingers 10 of the grating reflector 9 is set in therange of 0.05 to 0.075 with respect to a wavelength of SAW, and the dutyratio (w/p) of a portion of a part of the electrode fingers 10 thereofis set to the value ranging from 0.6 to just below 1.0.

[Third Embodiment]

As shown in FIG. 8, the attenuation constant α takes a minimum valuewhen the cut angle θ is about 39° and, by setting the cut angle θ in therange of 36° to 42°, the attenuation constant α can be limited to 0.005(dB/λ). In addition, as shown in FIG. 10, the attenuation constant αtakes a minimum value when the cut angle θ is about 40° and, by settingthe cut angle θ in the range of 37° to 43°, the attenuation constant αcan be limited to 0.005 (dB/λ).

As shown in FIGS. 9 and 11, when the cut angle θ is set in the range of36° to 43°, and the duty ratio (w/p) is 0.6 or higher, theelectromechanical coupling coefficient k² is larger than that in theconventional case in which the cut angle θ is 36° and the duty ratio(w/p) is 0.5. In the case having a large standardized electrodethickness (h/λ) shown in FIG. 11, compared with that shown in FIG. 9,the electromechanical coupling coefficient k² is larger under equal cutangles θ and equal duty ratios (w/p).

As described above, according to the third embodiment, the cut angle θof the LT substrate is set in the range of 36° to 43°, the electrodethickness of each of the electrode fingers 3 and 10 respectively of theinput and output side IDT 5 and 6 and the grating reflector 9 is set inthe range of 0.075 to 0.1 with respect to a wavelength of SAW, and theduty ratio (w/p) of the electrode fingers 3 and 10 respectively of theinput and output side IDT 5 and 6 and the grating reflector 9 is set tothe value ranging from 0.6 upto just below 1.0. Thus, it is possible torealize an acoustic wave apparatus with lower loss characteristics andwider band characteristics as compared with the conventional acousticwave apparatus of such a type.

Such an advantage can be obtained not only when the input and outputside IDT 5 and 6 and the grating reflector 9 all satisfy the foregoingconditions, but also when only one of the components satisfies theconditions.

For example, a similar advantage can be obtained when only the inputside IDT 5 satisfies the conditions, i.e., when the cut angle θ of theLT substrate is set in the range of 36° to 43°, the electrode thicknessof the electrode finger 3 of the input side IDT 5 is set in the range of0.075 to 0.1 with respect to a wavelength of SAW, and the duty ratio(w/p) of the electrode finger 3 of the input side IDT 5 is set to thevalue ranging from 0.6 to just below 1.0.

Similarly, a similar advantage can be obtained when only the output sideIDT 6 satisfies the conditions, i.e., when the cut angle θ of the LTsubstrate is set in the range of 36° to 43°, the electrode thickness ofthe electrode finger 3 of the output side IDT 6 is set in the range of0.075 to 0.1 with respect to a wavelength of SAW, and the duty ratio(w/p) of the electrode finger 3 of the output side IDT 6 is set to thevalue ranging from 0.6 to just below 1.0.

Likewise, a similar advantage can be obtained when only the gratingreflector 9 satisfies the conditions, i.e., when the cut angle θ of theLT substrate is set in the range of 36° to 43°, the electrode thicknessof the electrode finger 10 of the grating reflector 9 is set in therange of 0.075 to 0.1 with respect to a wavelength of SAW, and the dutyratio (w/p) of the electrode finger 10 of the grating reflector 9 is setto the value ranging from 0.6 to just below 1.0.

In addition, the foregoing advantage can be obtained not only when allthe electrode fingers of the input and output side IDT 5 and 6 and thegrating reflector 9 satisfy the conditions, but also when a part of theelectrode fingers satisfies the conditions.

For example, a similar advantage can be obtained when a part of theelectrode fingers 3 of the input side IDT 5 satisfies the conditions,i.e., when the cut angle θ of the LT substrate is set in the range of36° to 43°, the electrode thickness of a part of the electrode fingers 3of the input side IDT 5 is set in the range of 0.075 to 0.1 with respectto a wavelength of SAW, and the duty ratio (w/p) of a part of theelectrode fingers 3 thereof is set to the value ranging from 0.6 to justbelow 1.0.

Similarly, a similar advantage can be obtained when a part of theelectrode fingers 3 of the output side IDT 6 satisfies the conditions,i.e., when the cut angle θ of the LT substrate is set in the range of36° to 43°, the electrode thickness of a part of the electrode fingers 3of the output side IDT 6 is set in the range of 0.075 to 0.1 withrespect to a wavelength of SAW, and the duty ratio (w/p) of a part ofthe electrode fingers 3 thereof is set to the value ranging from 0.6 tojust below 1.0.

Likewise, a similar advantage can be obtained when a part of theelectrode fingers 10 of the grating reflector 9 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 36° to 43°, the electrode thickness of a part of the electrodefingers 10 of the grating reflector 9 is set in the range of 0.075 to0.1 with respect to a wavelength of SAW, and the duty ratio (w/p) of apart of the electrode fingers 10 thereof is set to the value rangingfrom 0.6 to just below 1.0.

In addition, the foregoing advantage can be obtained not only when allportions of a part of the electrode fingers of the input and output sideIDT 5 and 6 and the grating reflector 9, but also when only a portionthereof satisfies the conditions.

For example, a similar advantage can be obtained when only a portion ofa part of the electrode fingers 3 of the input side IDT 5 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 36° to 43°, the electrode thickness of a portion of a part ofthe electrode fingers 3 of the input side IDT 5 is set in the range of0.075 to 0.1 with respect to a wavelength of SAW, and the duty ratio(w/p) of a portion of a part of the electrode fingers 3 thereof is setto the value ranging from 0.6 to just below 1.0.

Similarly, a similar advantage can be obtained when only a portion of apart of the electrode fingers 3 of the output side IDT 6 satisfies theconditions, i.e., when the cut angle θ of the LT substrate is set in therange of 36° to 43°, the electrode thickness of a portion of a part ofthe electrode fingers 3 of the output side IDT 6 is set in the range of0.075 to 0.1 with respect to a wavelength of SAW, and the duty ratio(w/p) of a portion of a part of the electrode fingers 3 thereof is setto the value ranging from 0.6 to just below 1.0.

Likewise, a similar advantage can be obtained when only a portion of apart of the electrode fingers 10 of the grating reflector 9 satisfiesthe conditions, i.e., when the cut angle θ of the LT substrate is set inthe range of 36° to 43°, the electrode thickness of a portion of a partof the electrode fingers 10 of the grating reflector 9 is set in therange of 0.075 to 0.1 with respect to a wavelength of SAW, and the dutyratio (w/p) of a portion of a part of the electrode fingers 10 thereofis set to the value ranging from 0.6 to just below 1.0.

The invention has been described by taking the example of themode-coupled SAW resonant filter shown in FIG. 12. However, theinvention is not limited to such, and a similar advantage can beobtained even when a given number of IDT units other than 3 is prepared.Further, the invention is similarly advantageous even when it is appliedto a so-called transversal filter having a multielectrode structure orthe simply structured SAW filter shown in FIG. 2.

In addition, in the foregoing cases, the arraying cycles of theelectrode fingers 3 were all similar. However, the invention issimilarly advantageous even when the arraying cycles are partially orentirely changed. A similar advantage can also be obtained even in thecase where there is a floating electrode in the IDT or floatingelectrodes present at different positions in the IDT are electricallyconnected to each other.

Furthermore, the present invention is advantageous not only for the SAWfilter but also for all the other types of SAW devices including IDThaving a function of conversion for the electric signal of a oneterminal pair SAW resonator, a SAW delay line, a SAW dispersed delayline, a SAW convolver or the like with LSAW and SSBW. The invention isalso advantageous for all acoustic wave apparatus using such SAWdevices.

Industrial applicability

As apparent from the foregoing, the acoustic wave apparatus of theinvention is suitable for realizing a characteristic of smaller loss anda wider band than those in the conventional art.

What is claimed is:
 1. An acoustic wave apparatus comprising: apiezoelectric substrate mainly containing lithium tantalate; aninterdigital transducer including a conductor formed on said substrate;and a reflector including a conductor formed on said substrate, whereina surface rotated in a range of 34° to 41° from a crystal Y axis arounda crystal X axis of the lithium tantalate is set as a surface of saidsubstrate, a standardized electrode thickness (h/λ) obtained bystandardizing a thickness h of an electrode finger constituting at leasta part of said reflector by a wavelength λ of a surface acoustic wave isset in a range of 0.01 to 0.05, and a duty ratio (w/p) of the electrodefinger decided based on a width w and an arraying cycle p of theelectrode finger is set to the value ranging from 0.6 to just below 1.0.2. An acoustic wave apparatus comprising: a piezoelectric substratemainly containing lithium tantalate; an interdigital transducerincluding a conductor formed on said substrate; and a reflectorincluding a conductor formed on said substrate, wherein a surfacerotated in a range of 35° to 42° from a crystal Y axis around a crystalX axis of the lithium tantalate is set as a surface of said substrate, astandardized electrode thickness (h/λ) obtained by standardizing athickness h of an electrode finger constituting at least a part of saidreflector by a wavelength λ of a surface acoustic wave is set in a rangeof 0.05 to 0.075, and a duty ratio (w/p) of the electrode finger decidedbased on a width w and an arraying cycle p of the electrode finger isset to the value ranging from 0.6 to just below 1.0.
 3. An acoustic waveapparatus comprising: a piezoelectric substrate mainly containinglithium tantalate; an interdigital transducer including a conductorformed on said substrate; and a reflector including a conductor formedon said substrate, wherein a surface rotated in a range of 36° to 43°from a crystal Y axis around a crystal X axis of the lithium tantalateis set as a surface of said substrate, a standardized electrodethickness (h/λ) obtained by standardizing a thickness h of an electrodefinger constituting at least a part of said reflector by a wavelength λof a surface acoustic wave is set in a range of 0.075 to 0.1, and a dutyratio (w/p) of the electrode finger decided based on a width w and anarraying cycle p of the electrode finger is set to the value rangingfrom 0.6 to just below 1.0.
 4. An acoustic wave apparatus comprising: apiezoelectric substrate mainly containing lithium tantalate; aninterdigital transducer including a conductor formed on said substrate;and a reflector including a conductor formed on said substrate, whereina surface rotated in a range of 34° to 41° from a crystal Y axis arounda crystal X axis of the lithium tantalate is set as a surface of saidsubstrate, a standardized electrode thickness (h/λ) obtained bystandardizing a thickness h of a part of an electrode fingerconstituting a part of said reflector by a wavelength λ of a surfaceacoustic wave is set in a range of 0.01 to 0.05, and a duty ratio (w/p)of a part of the electrode finger decided based on a width w and anarraying cycle p of a part of the electrode finger is set to the valueranging from 0.6 to just below 1.0.
 5. An acoustic wave apparatuscomprising: a piezoelectric substrate mainly containing lithiumtantalate; an interdigital transducer including a conductor formed onsaid substrate; and a reflector including a conductor formed on saidsubstrate, wherein a surface rotated in a range of 35° to 42° from acrystal Y axis around a crystal X axis of the lithium tantalate is setas a surface of said substrate, a standardized electrode thickness (h/λ)obtained by standardizing a thickness h of a part of an electrode fingerconstituting a part of said reflector by a wavelength λ of a surfaceacoustic wave is set in a range of 0.05 to 0.075, and a duty ratio (w/p)of a part of the electrode finger decided based on a width w and anarranging cyce of a part of the electrode finger is set to the valueranging from 0.6 to just below 1.0.
 6. An acoustic wave apparatuscomprising: a piezoelectric substrate mainly containing lithiumtantalate; an interdigital transducer induding a conductor formed onsaid substrate; and a reflector including a conductor formed on saidsubstrate, wherein a surface rotated in a range of 36° to 43° from acrystal Y axis around a crystal X axis of the lithium tantalate is setas a surface of said substrate, a standardized electrode thickness (h/λ)obtained by standardizing a thickness h of a part of an electrode fingerconstituting a part of said reflector by a wavelength λ of a surfaceacoustic wave is set in a range of 0.075 to 0.1, and a duty ratio (w/p)of a part of the electrode finger decided based on a width w and anarraying cycle p of a part of the electrode finger is set to the valueranging from 0.6 to just below 1.0.